KR20090061833A - Activation method of mea using cyclo voltammetry - Google Patents

Abstract

An activation method of MEA(membrane-electrode assembly) using cyclo voltammetry is provided to accelerate activation of MEA, to reach maximum performance within fast time, and to improve performance of the membrane electrode assembly. An activation method of MEA(membrane-electrode assembly) using cyclo voltammetry comprises the steps of supplying humidified gas for hydration of a membrane and an electrode; and performing cyclo voltammetry activation method for an electrode layer activation. The first step does not use an electron load and application device and supplies only humidified air. The second step circulates voltage with an arbitrary value at voltage of 0~3 V in cyclo voltammetry cycle.

Description

Activation method of MEA using cyclo voltammetry

The present invention relates to a method for activating a polymer electrolyte fuel cell MEA, and more particularly, a polymer electrolyte fuel cell MEA (membrane / electrode assembly) using a cyclo voltammetry (CV) activation method capable of measuring maximum cell performance in a short time. It is about how to activate.

In general, polymer electrolyte fuel cells (Polyer Electrolyte Membrane Fuel Cells) are more efficient, higher current density and higher power density than other types of fuel cells.

In addition, the startup time is short and there is a fast response to load changes.

In particular, they are less sensitive to changes in pressure of the reaction gas and can produce a wide range of outputs.

Therefore, it can be applied to various fields such as power source of pollution-free vehicle, power generation for self, mobile and military power.

A polymer electrolyte fuel cell is a device that generates electricity while generating water by electrochemically reacting hydrogen and oxygen.

The supplied hydrogen is separated into hydrogen ions and electrons in the catalyst of the cathode electrode, and the separated hydrogen ions are passed to the anode through the electrolyte membrane.

At this time, the oxygen supplied to the anode combines with the electrons introduced into the anode through an external conductor to generate water while generating water.

At this time, the theoretical potential is 1.23V, the reaction equation is as follows.

^{_{Anode: H 2 → 2H + +}} 2e -

^{_{Cathode: 1/2 O 2 + 2H +}} + 2e - → H 2 O

In general, an electrode of a fuel cell is made by mixing a hydrogen ion carrier such as Nafion and a catalyst such as platinum. The activity of the fuel cell is reduced in electrochemical reaction during initial operation after fabrication of the fuel cell MEA.

The reason for this is as follows.

1. The flow path of the reactants is blocked and cannot reach the catalyst.

2. This is because the hydrogen ion carrier which forms three phase interface like catalyst is not easily hydrolyzed at the beginning of operation.

3. It is because the continuous mobility of hydrogen ions and electrons is not secured.

4. There is a possibility of containing impurities in electrode production. These impurities reduce the activity of the catalyst.

5. The oxide film formed on the catalyst reduces the activity of the catalyst.

6. There is no optimization of catalytic electronic structure suitable for fuel cell reactions.

For this reason, a procedure called activation is required to maximize performance after assembling the fuel cell.

The purpose of activation, also called pre-conditioning and break-in, is to:

1) Activate catalysts that do not participate in the reaction

2) Hydrogen membrane and electrolyte contained in the electrode are sufficiently hydrated to secure hydrogen ion passage

3) Toxic removal of catalyst

4) Removal of unnecessary oxide film surrounding the catalyst

5) conversion to catalytic electronic structures suitable for fuel cell reactions

Activation performed to reach maximum performance after fuel cell assembly may take hours or days depending on the operating conditions, and the fuel cell may operate without reaching the highest performance due to improper activation.

This improper activation procedure reduces the production rate in fuel cell mass production, results in the use of large amounts of hydrogen, increases the stack cost, and maintains low stack performance.

In addition, it takes a long time to measure the maximum cell performance of the MEA, which is difficult to measure in reality.

In this case, the MEA's maximum cell performance may be measured incorrectly and the development direction of the MEA may be changed.

Therefore, it is very urgent to devise an appropriate activation method for measuring the maximum cell performance of the MEA and stack in a short time.

Although activation of fuel cells is carried out in various ways by fuel cell manufacturers, the main activation method is to operate for a long time under a constant voltage.

AISIN SEIKI Co. Ltd. Patent (Application No .: 2003-143126, Activation method of solid polymer fuel cell, Japan) discloses a method of activating to a portion where the stack performance is no longer improved at low voltage for a long time. It takes a very long time to get the best performance from the battery.

In the Hyundai Motor patent (Application No .: 2005-127743, a new activation procedure of a polymer electrolyte fuel cell applying a step voltage operation) shown in FIG. 1, a voltage cycle is applied to a stack to activate the activation time at high relative humidity and temperature. It is suggested to reduce the average time to 4 hours.

However, this also has a disadvantage that takes longer than 8 hours when the activation is slow.

This conventional activation procedure aims at activating a catalyst that does not participate in the reaction, which is the purpose of activation, and 2) hydrating the electrolyte contained in the electrolyte membrane and the electrode sufficiently to secure a hydrogen ion channel, thereby removing impurities in the catalyst. And by removing the unnecessary oxide film and controlling the electronic structure, the method of increasing the activity is insufficient.

In order to increase the activity of the catalyst itself, a new MEA (membrane electrode assembly) activation procedure is required for 1) removal of catalyst poisons, 2) removal of unnecessary oxide films surrounding the catalyst, and 3) control of the catalyst electronic structure suitable for fuel cell reactions. It is true.

The present invention provides a MEA activation method for improving the performance of a polymer electrolyte fuel cell MEA (membrane electrode assembly) and stable performance in a short time.

Conventional MEA activation methods focus only on hydrating the electrolyte membrane and the electrolyte contained in the electrode, whereas CV activation method 1) removes poisoning of catalyst, 2) removes unnecessary oxide film surrounding catalyst, 3) MEA activation method through catalytic electronic structure control suitable for fuel cell reaction.

In the case of the MEA with the CV activation method, not only the maximum performance can be reached within a shorter time than the MEA without the performance, but also the performance increase is shown in some cases.

The method for activating a polymer electrolyte fuel cell MEA using the CV activation method according to an embodiment of the present invention for achieving the above object is to activate the fuel cell to stably measure the maximum performance of the MEA and the stack within about 2 and a half hours. After the installation to the equipment to perform the subdivided into two steps to activate, the first step to supply a humidified gas for the electrolyte hydration of the membrane and electrode, and the CV (cyclo voltammetry) activation method for the electrode layer activation Characterized in that the two steps to perform.

Here, in the first step, only humidified air can be supplied without using an electronic load and an application device, and in the CV cycle of the second step, the voltage can be circulated at an arbitrary value at a voltage between 0 V and 3 V, and 2 In a step, the CV cycle can be divided into several steps, or several CV cycles can be driven in succession at one time.

When dividing the CV cycle in step 2, the humidified air can be blown in the middle of the dividing.

In the first stage, the humidified gas may be humidified including nitrogen, all of oxygen, hydrogen, and an inert gas.In the second stage, the humidified gas may supply hydrogen at the cathode, and at the anode, at the CV cycle. Inert gas and oxygen can be supplied.

In order to activate the MEA and the stack, cells may be connected in parallel to each other in a CV cycle to control the stack, or cells may be connected in series to perform the entire stack in a CV cycle.

The method of activating the polymer electrolyte fuel cell MEA using the CV activation method provided by the present invention can activate the stack and the MEA within a short time to measure the maximum cell performance, and also minimize the cost of activation.

The problem with traditional activation is that it takes a lot of time, fuel, and 2. time, so there is a practical difficulty in measuring the MEA maximum performance, which in turn can incorrectly measure the maximum cell performance of the MEA.

Therefore, in the present invention, by performing the activation by subdividing the two-step process, it is possible to accelerate the MEA activation, there is an advantage that can measure the maximum cell performance.

In general, a fuel cell is a device that generates water by high-efficiency electrical energy and reaction by causing an electrochemical reaction inside by supplying air or oxygen to the anode and hydrogen or hydrogen to the cathode.

The electrochemical reaction by the reaction gas occurs in the catalyst layer inside the fuel cell, and the generated hydrogen ions move through the electrolyte and the electrolyte membrane in the catalyst layer, and the electrons are transferred to the electricity generating device through the catalyst, the gas diffusion layer, and the separator. Will enter.

However, since hydrogen ions moving through the electrolyte or the electrolyte membrane move through the water present in the electrolyte membrane, the electrolyte and the electrolyte membrane in the catalyst layer must be sufficiently hydrated in order for the fuel cell to exhibit better performance.

In addition, for the electrochemical reaction, the reaction gas must reach the catalyst layer smoothly.

In addition, in order to achieve maximum cell performance, an electronic structure conversion of a catalyst suitable for removing unnecessary oxide films and impurities and reactions in a catalyst layer that may occur during manufacturing and storage should be performed.

Thus, the purpose of activation is as follows.

1) Activate catalysts that do not participate in the reaction

2) Hydrogen membrane and the electrolyte contained in the electrode are sufficiently hydrated to secure hydrogen ion passage

3) Toxic removal of catalyst

4) Removal of unnecessary oxide film surrounding the catalyst

5) Catalyst electronic structure control suitable for fuel cell reaction

The present invention finds a fast and complete activation condition of a fuel cell that can satisfy the above conditions, and provides an activation method for expressing optimal fuel cell performance.

The present invention seeks to present an accelerated activation procedure for reliably measuring maximum performance within about two and a half hours for MEAs and stacks that require long activation.

First, the fuel cell is mounted on the equipment for activation of the fuel cell, the reaction gas is supplied to the fuel cell, the fuel cell is maintained at no load or load state, and the preparation process is divided into two steps. do.

The two steps are as follows:

▶ Step 1: Supply humidified nitrogen → Method for electrolyte hydration of membrane and electrode

▶ Step 2: CV (cyclo voltammetry) → electrode layer activation method

In the first step, humidified nitrogen is supplied to hydrate the membrane and the electrode.

Humidified nitrogen supplies water to the electrolyte in the membrane and electrode.

The water thus supplied secures a hydrogen ion passage between the membrane and the electrolyte of the electrode, thereby facilitating the movement of the hydrogen ions generated at the cathode.

Here, in the first step, only humidified air may be supplied without using an electronic load and an application device.

In this case, the gas provided for humidification may include all of nitrogen, oxygen, hydrogen, and an inert gas.

However, only this humidification does not completely activate the MEA.

During the activation process, an electronic structure control suitable for removing impurities and unnecessary oxide film and reacting should be made.

In the present invention, a two-step CV step is introduced to easily perform electronic structure control suitable for removing impurities and unnecessary oxide films and for reaction.

In the CV stage, hydrogen is supplied from the cathode, nitrogen is supplied from the anode, and the voltage is repeatedly circulated in a specific region within 0.0V to 3V.

In this process, the impurities of the catalyst and the unnecessary oxide film are converted into the electronic structure of the catalyst suitable for removal and reaction.

The reason for going through the CV process is that it is effective in removing impurities and unnecessary oxide film.

In order to remove impurities and unnecessary oxide film, a constant energy must be supplied.

The energy commonly used to remove these impurities and unwanted oxide films is thermal energy.

However, the thermal energy required to remove unnecessary oxide film and impurities is 300 degrees or more, at which time the electrolyte is also thermally decomposed to the film and the electrode.

Therefore, there is a limit in removing impurities and oxide films by using thermal energy.

To overcome these limitations, other energy sources must be used.

In the present invention, electrochemical energy was used to remove impurities and oxide films.

Oxidation must be done to remove impurities.

Oxidation is a reaction that loses electrons.

The loss of electrons works better at higher voltages.

In addition, the removal of the unnecessary oxide film is a reduction reaction.

The reduction reaction is a reaction for obtaining electrons.

This reduction reaction is easier to remove the lower the voltage.

That is, the potential must be high to remove impurities, and the potential must be lowered to remove unnecessary oxide films.

Thus, in order for redox to occur, the oxidation potential must be made high and the reduction potential must be made low.

This is done in the CV cycle.

Therefore, CV cycle is the process of repeatedly circulating voltage in a specific region within 0.0V to 3V.

That is, in the step of increasing the voltage from 0V to 1V or more, impurities are smoothly removed because the oxidation potential is 1V or more, and in the step of decreasing the voltage to 0V at a voltage of 1V or more, the reduction potential is sufficiently low near 0V so that the oxide film is smoothly removed. do.

Therefore, unnecessary oxide film and impurities can be removed while repeating this voltage.

In this two-step CV cycle, you can cycle the voltage at any value at a voltage between 0.0 V and 3 V, divide it into several steps, or drive several CV cycles in succession at once. have.

In addition, when the CV cycle is divided, humidified air may be blown in the middle of the division. The humidified gas may supply hydrogen at the cathode and inert gas and oxygen at the anode.

However, in the conventional step voltage method, hydrogen is injected into the cathode and air is injected into the anode to generate a current at a low voltage, thereby removing impurities and unnecessary oxide film.

In this case, the lower the reduction potential, the more the oxide film is removed. On the other hand, this method has a limit of removing the oxide film because the total voltage is 0.4V and the reduction potential is high.

In addition, in order to oxidize the impurity, the voltage must be high, but since the maximum is 1V or less, which is the OCV of the MEA, there is a limit in removing the impurity.

2 shows the cell performance according to the first stage and the number of CV cycles.

In order to determine the cell performance according to the electrolyte humidification, which is the first step, the cell performance was measured after the initial 30 minutes of humidification. As a result, the cell performance was very low due to insufficient humidification.

However, the cell performance measured by performing humidification for 2 hours and the cell performance measured by performing humidification for 3 hours were almost similar.

These results indicate that humidification alone is limited to increase cell performance, and activation of the catalyst, which is a two-step process, is essential.

In FIG. 2, the cell performance increases with the number of CVs and reaches a constant value.

Therefore, the average number of cycles of CV activation is about 30 to 45 times.

The change in electrode activity according to the CV cycle is shown in FIG. 3.

When no CV cycle is performed, the current increases with voltage between 0.2V and 0.6V.

This is because hydration of the catalyst site occurs due to the water generated in the catalytic reaction by the initial CV cycle, thereby reducing the resistance at the electrode.

In addition, it was confirmed that the Pt-oxide layer change occurred at 0.8V to 1.2V by the CV cycle.

This may be due to the removal of unnecessary Pt-oxide layers and impurities.

4 illustrates the cell performance of the activation process by the CV activation method and the cell performance of the step voltage method.

The cell performance of the activation using only the CV activation method was similar to that of the activation using the 4-hour step voltage method.

This means that MEA activated by CV activation shows the maximum cell performance.

In FIG. 5, after performing the step voltage method for 6 hours, the CV activation method was further performed to show each cell performance.

The performance was 749.8 mW / cm ² for 6 hours using the step voltage method.However, when the CV activation method was used, the cell performance increased gradually with CV cycles. ² increased by 18%.

4 and 5, the step voltage method performed for a long time has a limitation in activating the fuel cell, and as a result of performing the activation by the CV activation method, it is confirmed that the maximum cell performance is reached in a short time. Could.

Therefore, when applying the activation technique using the CV activation method provided by the present invention,

1) Fuel cell activation time can be reduced by at least 50%.

2) When activating the MEA, in step 1, only the humidification gas is supplied without using the activation equipment. In step 2, the electronic accreditation system can be used up to 20 minutes. The cost to the device can be significantly lowered.

3) Compared with about 1,200L / cell of hydrogen used in the existing activation procedure, hydrogen consumption can be reduced to less than ¼ or less with the amount of hydrogen used below 300L / cell, which is necessary for CV cycle, which can significantly reduce the stack production cost. have.

1 is a graph showing an activation evaluation method using a step voltage and a result

Figure 2 is a graph showing the cell performance according to the number of CV cycles and when only the humidification step 1 process of the present invention

3 is a graph showing the change in electrode activity according to the CV cycle, a two-step process of the present invention

4 is a graph showing the cell performance of the activation process by the CV activation method and the cell performance of the step voltage method introduced.

5 is a graph showing the performance of each cell by further performing the CV activation method after performing the step voltage method for 6 hours.

Claims (8)

In a method of activating a polymer electrolyte fuel cell MEA, After the fuel cell is mounted on the equipment to activate the fuel cell, the fuel cell is subdivided into two stages, and the first stage is supplied with humidified gas for electrolyte hydration of the membrane and electrode, and the CV (cyclo voltammetry) for the electrode layer activation. A method of activating a polymer electrolyte fuel cell MEA using a CV activation method, characterized in that performing two steps of the activation method. The method of claim 1, wherein in the first step, humidified air is supplied without using an electronic load and an application device. The method according to claim 1 or 2, wherein the CV cycle of the second step circulates the voltage at any value at a voltage between 0V and 3V. The method of claim 3, wherein the CV cycle is divided into several steps in the second step, or several CV cycles are continuously driven at a time. 5. The method of claim 4, wherein when the CV cycle is divided in the second step, humidified air is blown in the middle of the division. The method of claim 1 or 2, wherein the gas humidified in the first step is humidified by including oxygen, hydrogen, and an inert gas, including nitrogen, and the method of activating the polymer electrolyte fuel cell MEA using the CV activation method. . 3. The polymer electrolyte fuel cell MEA of claim 1, wherein the humidified gas is supplied with hydrogen at the cathode and inert gas and oxygen is supplied to the anode during the CV cycle. Activation method. The CV activation according to claim 1 or 2, wherein in order to activate the MEA, cells are connected in parallel to each other in a CV cycle to control stacks, or cells are connected in series to perform a stack as a CV cycle. Method for activating a polymer electrolyte fuel cell MEA using the method.

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